Right on, there is no such thing as too long, just you are wasting energy.
Now, if you could rig up thermometer, read the temp of the engine, plug in the block-heater, check every 15 min and when it reaches max., you now know how long to run the timer or plug the heater in for.
Judging for the weather, just adjust the time up or down depending on outside temperature.
Probably find 2 hours is very close to max and less time if it is warmer outside.